This invention relates to motor controllers and, more particularly, to a motor soft starter that automatically senses if the starter is bypassed and adds time delay to end of a running operation to prevent arcing.
Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. One type of starter/controller is a reduced voltage solid state starter (RVSS). The starter/controller, referred to hereinafter as simply a controller, uses SCRs, or triacs, for controlling application of AC line voltage to the motor. The controller typically includes aluminum finned heat sinks associated with the SCRs for dissipating heat. When the controller has been running for an extended period of time, the heat sinks and SCRs become warm. Normally, the heat sinks dissipate this heat to the air surrounding the controller. A housing enclosing the controller has ventilation openings to exchange cooler air from outside with air around the controller which has been warmed. If the air is not exchanged, then the heat will build up inside the housing and damage or destroy the SCRs.
In some installations, the housing can not be ventilated due to the surrounding environment. For example, in a dairy the equipment must be washed with a high pressure hose. Cleaning solutions would be forced through the ventilation openings and damage or destroy the SCRs. In this type of application the housing must be completely sealed. The heat build up must be controlled since it cannot be vented to the outside. One known way to control this heat is to bypass the controller during its running time.
To bypass the controller, an electro mechanical contact is used in parallel with each pair of SCRs. The contacts close after the controller motor load has completed starting and is running at full speed. When the contacts close, the current stops flowing through the SCRs and starts flowing through the contacts. This is because the contacts have lower impedance than the SCRs. With the contacts carrying the current at lower impedance, the heat is virtually eliminated compared to that dissipated by the SCRs. Therefore, no ventilation is required.
At the end of the motor running operation, the motor is stopped by breaking the circuit to the motor load. The contacts are carrying the current at this time. A contact arcs when it opens with current flowing through it. This arcing exceeds the blocking capability of the parallel SCRs and can damage or destroy the SCRs. To prevent this damage, the SCRs must stay ready to conduct current as soon as the contacts open. This prevents arcing from occurring since the motor load current is still flowing. After the contacts have fully opened, so that they cannot carry current, the SCRs may be turned off. Since the SCRs turn off at zero current, there is no energy in the controller motor load and no arcing occurs to damage or destroy the triacs.
To control the turn offofthe triacs under the above-described conditions, a time delay relay is currently used. Since this delay may not be desirable for all applications, the equipment user or equipment manufacturer must remember to activate this delay when needed.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention there is provided a control which automatically senses bypassing of the solid state starter control switches.
Broadly, there is disclosed herein a motor controller for a motor operated by an AC line voltage. The controller includes solid state switch means connectable between an AC line and a motor terminal for controlling application of AC line voltage to the motor. Bypass means are connected in parallel with the solid state switch means for selectively bypassing the solid state switch means. Sensing means sense if the bypass means is bypassing the solid state switch means.
It is a feature of the invention that the motor controller further includes control means connected to the sensing means and the solid state switch means for varying relative duration of on time and off time of the solid state switch means during each cycle of line voltage to control motor voltage at a reference level, the control means adding a time delay to a running operation prior to turning the solid state switch means off to stop the motor if the bypass means is bypassing the solid state switch means.
It is another feature of the invention that the switch means comprises a silicon controlled rectifier (SCR).
It is a further feature of the invention that the switch means comprises a triac.
It is yet another feature of the invention that the sensing means senses if the bypass means is bypassing the solid state switch means by sensing if there is a voltage pulse across the switch means during time between conduction of the solid state switch means in each half cycle.
It is still a further feature of the invention that the sensing means senses voltage across one of the solid state switch means and motor current.
It is an additional feature of the invention that the sensing means measures sensed voltage during time between conduction of the solid state switch means.
There is disclosed in accordance with another aspect of the invention a motor controller for a motor operated by an AC line voltage. The motor controller includes SCRs connectable between an AC line and a motor terminal for controlling application of AC line voltage to the motor. Contacts are connected in parallel with the SCRs for selectively bypassing the SCRs. A sensing circuit senses if the contacts are bypassing the SCRs.
There is disclosed in accordance with a further aspect of the invention a method of automatically sensing bypassing of a motor controller for a motor operated by an AC line voltage. The method comprises providing SCRs connectable between an AC line and a motor terminal for controlling application of AC line voltage to the motor; selectively bypassing the SCRs using contacts connected in parallel with the SCRs during a run mode of the motor; and automatically sensing if the contacts are bypassing the SCRs.